The Introduction dives straight in to the basic premise, that ancient and insane deities exist and are still trying to invade Earth and that someone has to stop them, whatever the cost to life and sanity. It then moves on to the burning question: there's already a Call of Cthulhu RPG dealing with just that, so why a new game? The answer lies in the Gumshoe ruleset, developed by Pelgrane Press for the purpose of running games based around investigation and discovery, and built so that any adventure depending on certain clues being found will have those clues found! It's designed for people - Keepers and players alike - who want to concentrate on figuring out what the clues mean, rather than having to wonder if they actually have all the clues. This game also aims to enable two styles of play - the Purist style of intellectual analysis which enjoys watching the horror unfold knowing that it will end in madness; and the Pulp style which allows for a more physical approach, value the actual struggle against evil... and pays a bit more regard to character survival. The best games mix a bit of both - certainly Lovecraft's writing did! - but as parts of the rules favour one or the other style, they are marked so players can choose the bias they prefer, if any.

So, on to Chapter 1: The Investigator. That's your character, and creating him begins with the starting point that he did something else for his living (indeed, may still do) before starting to investigate the horrors of the Cthulhu Mythos. Each one gives you certain skills and areas of knowledge, and a credit rating; choose based on what you want your character to be able to do in the game. Each occupation also confers an in-game advantage. There's a balanced note on gender, explaining that the 1930s weren't quite as sexist as many imagine at least in terms of what women did (although probably in reactions to them), and anyway if you are happy with tentacled monsters slithering around New York, why bat an eyelid at a female cop (and the NYPD had a few by then anyway!). There's also advice on how to design new occupations, or tweak existing ones, if players find none that suits what they have in mind.

Occupation chosen, the next stage is to select what motivates the character. A range of 'drives' are provided, and while their use is primarily role-playing rather than mechanical advantage, it is important to know just why you want to go sticking your nose into the Things That Should Not Be. Next, you need to select your abilities for which a point-buy system is used. Abilities are divided into investigative ones (listed under the chosen occupation) including all academic, interpersonal and technical skills; and general ones consisting of everything else including health, sanity and stability. Plenty of detail and discussion ensures that wise choices can be made.

Next, Chapter 2: Clues, Tests and Contests explains how the game works at a mechanical level. Being built as a specialist investigatory rules system, Gumshoe makes the actual acquiring of clues simple, it is pieceing them together to work out what is going on and then determining a course of action to deal with it where the real challenge lies. To get the information needed, characters just have to visit locations, talk to people, read books and so on. Provided they look in the right place and have the right abilities they will get the clues, although points may be spent to get extra detail out of them. However, there are times when the outcome is not certain, something which applies to general skills not investigative ones, and then a test needs to be made, a test in which there is a reasonable chance of failure. These generally happen when success (or otherwise) is dramatically important or if the task being attempted is exceptionally difficult... and it is recommended not to use this when the very adventure depends on managing to do that thing. Or at least, to present other options rather than a stark suceed/fail - perhaps instead of failing entirely to accomplish the task, the character manages it but injures himself in the process. The key concept is never to allow a failed die roll, or even players doing the 'wrong' thing to completely derail the game.

The chapter then moves on to discuss opposed tests, that is where the character is competing against someone else, whether in a game of poker, an academic debate, keeping his trap shut when the police what to know what is going on and, yes, when the talking stops and fists and bullets fly. Narrative description rather than mere arithmetic and die rolls is king, and the ultimate goal of solving the mystery that is the plot of the adventure must remain in mind. Next comes that all-important sanity and stability, after all the real threat here is to your character's mind rather than his body. Both stability and sanity relate to mental health, but stability operates in the short term being instant reactions to something that may be shocking or scary but is got over quickly, while sanity is a measure of how well your character is able to resist, even blank out, the awful truth of the Mythos. And this is where the mechanical side of a character's drive comes in - that fundemental motivation will oft times suggest a course of action that is risky or worse, but that drive is considered so strong that resisting it in a coldly-rational way costs you stability points... but doing something ostensibly foolish because it fits your motivation gains extra points. A neat mechanic to promote both role-playing and thought. There's plenty more as well, including some inspired and devious ways to make a character going insane actually play out around the gaming table. Fortunately, means of recovery and the restoration of mental health are also discussed.

The rest of the work is aimed mostly at the Keeper, beginning with Chapter 3: The Cthulhu Mythos. Naturally, the most powerful Great Old Ones are not going to be in the least bit bothered by anything the characters try, so the only stats they are given relate to the effect they have on the characters' stability and sanity. Still, they are listed here in all their contradictory glory - who among you dare demand that they be consistent? It is a convenient tool for the Keeper whose players already know (or think they know) about the Mythos from reading Lovecraft or playing other games: you can pick and choose not just which Great Old One is behind your adventure but which aspect of those presented here that you'd like to use... not to mention that the texts the characters find may well present contradictory information. Listing done, the discussion moves on to tomes and magic, things that the characters are rather more likely to encounter on a day-to-day basis than the Great Old Ones in person. Quite a few tomes are listed, it's up to the Keeper to decide the precise contents and usefulness in a given adventure. There are also guidance notes for those wishing to develop their own tomes, a practice to be encouraged. Next comes some spells - with the chilling note that, at least at first, the characters are more likely to be on the receiving end than casting them themselves.But sufficient perusal of those tomes may eventually see characters casting spells, so rules and details are provided. The spells are followed by a cast of creatures, minions and the like with which to plague the characters. For each, the sort of clues that might lead to them are given, classified by the investigation skill that would discern that clue. Not all adversaries go on four legs, tentacles, or the like; so the next section deals with cults and cultists, those who have chosen to venerate and serve the Great Old Ones. For each cult, there is background as well as ideas for how they may come to the attention of the characters. Naturally, creative Keepers will come up with cults of their own, and guidance is provided to that end.

Next, Chapter 4: The Thirties gives a feel for the era in which this game is set, the 1930s. Many aspects are touched upon, including those - such as 1930s' attitudes to people of different skin colours - which the modern mind finds uncomfortable. In all things, it is up to the group to decide whether they want to be authentic to the 1930s or to rewrite societal values in the light of present-day opinions. Poverty, famine, war (and the threat thereof), political movements and the rise of the movies - it's all there and readily researchable in as much detail as you want. Rich strands can be drawn from the politics of the time, yet while Nazis can be convenient baddies, what if the only way to stop a Mythos plot is to ally with them? Hard choices can make for good, even memorable, games. There's also a section describing likely locales for adventure, suitable for Pulp and Purist alike. This is followed by information about the technology, weapons, equipment and money available to the characters.

Scene set, Chapter 5: Putting it all Together looks at ideas for both players and Keepers in how best to approach this game, both as a game and from the standpoint of the alternate reality you're trying to create together. Many of the tips work well for any type of game, particularly one in which investigation plays a part, never mind this one, so read and apply them. There's even better advice for Keepers on how to both plan and present adventures. It's a game of investigative horror, so both the horror and the investigative process need to be thought through. While much of this is naturally angled towards game masters running the Gumshoe system and specifically this implementation of it, there is so much here that would benefit any game master whatever game they are running that I'm tempted to recommend getting this even if you don't fancy Cthuthuloid adventures!

Next, Chapter 6: Campaign Frames talks about moving beyond one-off adventures or even a series thereof, and building a coherent campaign with an overarching plotline in which individual adventures are components building in stages towards the ultimate finale. There are all manner of ideas and resources to empower the creative Keeper to achieve this end. There are additional rules, investigative abilities and more which should prove useful... even if you are not running an ongoing campaign, but especially good if you are. Thoughts on the creative use of recurring NPCs, friends and foes alike, weaving real-world history into your adventures and adding a Mythos twist... all manner of goodies here.

OK, enough theory, Chapter 7: The Kingsbury Horror brings it all all together with a ready-to-run adventure. Starting with a series of gruesome murders (which, as it happens, are historical although probably not really connected with the Cthulhu Mythos!), the characters have a chance to prove themselves better investigators than Eliot Ness, no less! Loads of people to talk to, interesting places to visit and a steady stream of clues provide not just a cracking good adventure, but an excellent example of how to put one together for this system as well.

Appendices cover such matters as melding this game with classic Call of Cthulhu, using either Gumshoe or Chaosium's Basic Roleplaying (BRP) system as preferred, and even how to convert characters between the two systems. There are sources and resources to help you get into both the Mythos and the 1930s, as well as other fiction of suitable style to be mined for ideas. There's not just a character sheet, but also some useful forms to enable the Keeper to keep track of things. Short biographies of the main contributors (who all seem to require strait jackets!) and comprehensive indices round the book off.

Overall, this is a masterful melding of the Gumshoe system with classic Cthulhu Mythos gaming, an inspired match. There's so much goodness in this that I'll be back again and again, not just to play but to mine for ideas whatever I am doing.